NEW YORK, March 14 (Reuters) - Wall Street ended lower on Friday with all three major stock indexes down for the week, as concerns over tensions between Ukraine and Russia escalated ahead of a referendum in Crimea this weekend.

The S&P 500 ended below a key technical support level of 1,850 for the second day. The index also ended down 2 percent for the week, its biggest weekly loss since late January.

Moscow shipped more troops and armor into Crimea on Friday and repeated its threat to invade other parts of Ukraine in response to violence in Donetsk on Thursday night despite Western demands to pull back.

Following the recent selloff in emerging markets, some market participants believe now is the time to get into emerging market equities, but analysts are wary.

“The numbers, certainly on the face of it, look compelling,” said Jade Fu, investment manager at Heartwood Investment Management in London. The MSCI emerging markets index is trading at 1.5 times price-to-book value and poor sentiment has already resulted in outflows of over $30 billion from emerging market equities this year, Fu said.

But “it is difficult to hold a very optimistic view of emerging market assets at this time, even if lower valuations have made them appear more attractive.”

The Dow Jones industrial average fell 43.22 points or 0.27 percent, to 16,065.67, the S&P 500 lost 5.21 points or 0.28 percent, to 1,841.13 and the Nasdaq Composite dropped 15.023 points or 0.35 percent, to 4,245.396.

For the week, the Dow fell 2.35 percent, the S&P dropped 1.97 percent and the Nasdaq lost 2.1 percent.

In economic news, producer prices fell 0.1 percent in February, dragged down by falling costs for services, offering little sign of inflation pressures. U.S. consumer sentiment weakened in early March as an unusually harsh winter appeared to dim views on the economy’s prospects.

General Mills Inc fell 2.4 percent to $49.77. It forecast third-quarter earnings below analysts’ expectations as it faces increased competition from store brands and spends more on marketing its yogurts.